1. Field of the Invention
This invention relates to the treatment of waste liquor from industrial processes and is more particularly concerned with the treatment of waste liquor containing a mixture of polysulfide and thiocyanate to recover sulfates therefrom.
2. Description of the Prior Art
Coke oven plants, associated with steel mills, generate gases which are rich in hydrogen cyanide, hydrogen sulfide and reduced sulfur compounds together with gaseous products of incomplete combustion such as carbon monoxide, hydrogen, nitrogen, methane, carbon dioxide and the like. The coke oven gases therefore have to be treated for removal of hydrogen sulfide and reduced sulfur compounds before these gases can be utilized for their fuel value in furnaces, otherwise the sulfur compounds would be converted to sulfur dioxide and discharged in the flue gas from the furnaces.
Currently the standard technique for treating coke oven gases has been the so-called Stretford process which employs a vanadate compound and a quinone to convert the hydrogen sulfide and reduced sulfur compounds in the gas to elemental sulfur on a substantially quantitative basis. Unfortunately, the hydrogen cyanide present in the gas is totally converted to thiocyanate, in the presence of elemental sulfur, if treated in the Stretford process. The thiocyanate is difficult to separate from the costly vandates and quinones; also it greatly complicates the recovery and re-use of the latter. Accordingly, the coke oven gas must be selectively treated to remove at least the major amount of the hydrogen cyanide before the gas can be subjected to the Stretford process. The hydrogen cyanide is generally separated in this manner by contacting the gas with a wash system containing sodium or ammonium polysulfide. The latter scrubs the hydrogen cyanide from the gas by reacting with it to form the corresponding sodium or ammonium thiocyanate. The hydrogen cyanide level in the treated gas is reduced to a sufficiently low level (ca 150 ppm from an initial level of 5000 ppm) to avoid any problems when the gas is subsequently subjected to the Stretford process; see Moyes and Vasan, Oil and Gas Journal Sept. 2, 1974. "Holmes-Stretford H.sub.2 S Removal process proved in use."
The waste liquor containing a mixture of sodium or ammonium polysulfides and thiocyanate which results from the hydrogen cyanide scrubbing process, is currently processed by evaporation followed by oxidation at high pressures and temperatures in an autoclave to convert the polysulfides and thiocyanates to the corresponding sulfates. However, this method involves high capital investment and operating costs and also encounters serious problems of equipment corrosion. It has now been found that the waste liquor from the above hydrogen cyanide scrubbing process can be processed very satisfactorily and at greatly reduced cost to yield commercially marketable grades of sodium or ammonium sulfate as well as yield useful combustible gaseous by-products.